Theory Behind Working

CDMA (Code Division Multiple Access) is based on the principle of DSSS (Direct Sequence Spread Spectrum), which is one of the prevailing SS (Spread Spectrum) Technologies in today’s world. Spread Spectrum:

In SS technique, the same bandwidth is shared by multiple users, without significantly interfering with each other. The spreading waveform is controlled by a Pseudo – Noise (PN) sequence, which is binary random sequence. This PN is then multiplied with the original baseband signal, which has lower frequency, which yields a spread waveform that has a noise like properties. In the receiver, the opposite happens - the passband signal is first demodulated, and then despread using the same PN waveform. An important factor here is the synchronization between the two great sequences.

Pseudo Noise (PN):

It is the key factor in DSSS systems.
A Pseudo – Noise or Pseudo – Random sequence is a binary sequence with an autocorrelation that resembles, over a period, the autocorrelation of a random binary sequence. It is generated using a shift Register, and a combinational logic. They have the following important properties:

· Balanced: The codes should be ``balanced'': The difference between ones and zeros in the code may only be 1. This last requirement stands for good spectral density properties (equally spreading the energy over the whole frequency-band)

· Single Peak auto-correlation function: The codes must have a sharp (1-chip wide) autocorrelation peak to enable codesynchronization. · Deterministic: The subscriber station must be able to

independently generate the code that matches the base station code. It must appear random to a listener without prior knowledge of the code

Process Gain:

The auto-correlation function of a random binary sequence is a triangular waveform as in the following figure, where TC is the period of one chip.

The spectral density of such a waveform is a Sinc function squared, with first zeros at ± 1/TC

Process Gain: Since multiplication in the time domain corresponds to convolution in the frequency domain, a narrow band signal multiplied by a wide band signal ends up being wide band. One way of doing this is to use a binary waveform as a spreading function, at a higher rate than the data signal. Here the three signals corresponds to x(t), p(t) and y(t) discussed below. The first two signals are multiplied together to give the third waveform. Bits of the spreading signal are called chips. In the figure shown below, Tb represents the period of one input data bit and Tc represents the period of one chip. The chip rate, 1/Tc, is often used to characterize a spread spectrum transmission system. The Processing Gain or sometimes called the Spreading Factor is defined as the ratio of the information bit duration over the chip duration:

Hence, it represents the number of chips contained in one data bit. Higher Processing Gain (PG) means more spreading. High PG also means that more codes can be allocated on the same frequency channel.